It is known in the art of smelting and purifying metals to introduce gas into molten metal to remove impurities. Specifically, when processing molten aluminum, it is desirable to remove dissolved gases, particularly hydrogen, and to remove dissolved metals, particularly magnesium. Those skilled in the art refer to removing dissolved gas from molten aluminum as "degassing," and refer to removing magnesium as "demagging." Nitrogen or argon is generally released into molten metal for degassing purposes while chlorine gas is generally used for demagging.
When demagging or degassing aluminum, gas is released into a quantity of molten aluminum, this quantity generally being referred to as a bath of molten aluminum. The bath is usually contained within the walls of a reverbatory furnace. The present invention can be used for either demagging or degassing purposes.
When demagging aluminum, chlorine is released into the bath and bonds, or reacts, with magnesium wherein each pound of magnesium reacts with approximately 2.92 pounds of chlorine to form magnesium chloride (MgCl.sub.2). Several methods for introducing chlorine into a molten aluminum bath are disclosed in the prior art. For example, it is known to introduce a flux containing chlorine into the bath, rather than introducing chlorine gas. Such a flux may contain a double salt of chlorine, such as CRYOLITE. It is also known to employ an apparatus whereby nitrogen or argon gas is introduced through a hollow rotating shaft utilizing an apparatus known as a rotary degasser. Another apparatus is a gas-injection system including a pump having a discharge, a metal-transfer conduit extending from the discharge and a gas-injection conduit connected to the top of, and extending into, the metal-transfer conduit. Molten aluminum is pumped through the metal-transfer conduit and gas is injected through the gas-injection conduit into the upper portion of the pumped molten metal moving through the metal-transfer conduit.
Other prior art includes: (a) a molten metal pump and gas-injection apparatus whereby gas is introduced through a tube into a passage and is released into molten metal entering the pump inlet; (b) a gas-treatment apparatus comprising: (i) a purification device, which is immersed in a molten metal bath contained within a furnace, and (ii) a decanting and degassing tank located outside of the bath; (c) U.S. Pat. No. 5,662,725 to Cooper entitled "System And Device For Removing Impurities From Molten Metal," which discloses an apparatus that releases gas into the bottom or sides of a moving molten metal stream so as to better disperse the gas within the stream (the disclosure of this issued patent is incorporated herein by reference).
Specific examples of prior-art devices are disclosed in U.S. Pat. No. 3,650,730 to Derham et al., U.S. Pat. No. 3,767,382 to Bruno et al., U.S. Pat. No. 4,169,584 to Mangalick, U.S. Pat. No. 4,351,314 to Koch, U.S. Pat. No. 4,003,560 to Carbonnel, and U.S. Pat. No. 5,203,681 to Cooper.
One problem with the known gas-injection or gas-release devices is often that the gas is released through an opening formed at the end of a gas-injection conduit that extends into the molten metal stream from the top of a metal-transfer conduit through which the molten metal is being pumped or otherwise conveyed. When the molten metal stream moving through the metal-transfer conduit contacts the gas-injection conduit, it is obstructed by and diverted around the end of the gas-injection conduit creating a low pressure zone behind the end of the gas-injection conduit. At least some of the gas released through the opening of the gas-injection conduit immediately enters this low pressure zone, rises to the inner surface of the top of the metal-transfer conduit and is not dispersed within the moving molten metal stream. Much of the injected gas, therefore, remains in contact with the top of the metal-transfer conduit until it exits the metal-transfer conduit, at which point it completely separates from the flowing molten metal and rises to the surface of the molten metal bath. Therefore, the gas is not effectively dispersed within the molten metal stream passing through the conduit, and the percentage of chlorine that actually bonds with magnesium to form MgCl.sub.2 is relatively low. As it will be appreciated by those skilled in the art, the greater the dispersion of gas within the molten metal stream, the greater the demagging efficiency because a higher number of molecules contact metal molecules, thus giving more molecules a chance to interact and bond to form MgCl.sub.2. Improving the efficiency of the demagging process is highly desirable. It reduces material costs because less chlorine gas is used. Furthermore, chlorine gas that does not bond with magnesium either bonds with aluminum to form aluminum bichloride, an undesirable contaminant, or rises to the top of the molten metal bath and escapes into the atmosphere, where it is an undesirable pollutant.